Firing furnaces for dental purposes have been known for a long time in which dental restoration parts are fired, sintered, debound or pressed taking into account special given firing, sintering, debinding and/or pressing parameters. In this connection it is also known that the quality of the produced dental restoration parts strongly depends on the compliance with the mentioned process parameters during the entire process.
Dental restoration parts are typically placed in the furnace individually or together on a firing chamber floor or base. For the firing process a firing hood that comprises the heating for the furnace, is combined with the firing chamber floor and is usually sealed off so that the start of the firing cycle is possible. In this connection it is advantageous if the firing hood itself is supported as to be able to be lifted and/or pivoted as vibrations of the firing chamber floor and vibrations of the dental restoration parts as a consequence thereof may then be reliably avoided.
As soon as the firing hood is closed, the actual firing, sintering, debinding or pressing process may be started wherein a hold time is typically included as a precaution.
As both in firing furnaces, sintering furnaces, debinding furnaces and pressing furnaces the materials used are sensitive to a noncompliance with the process parameters, in particular temperature and pressure, usually more sensors, in particular pressure sensors and/or temperature sensors are employed in order to detect the precise course of the process parameters and to intervene if necessary to perform a control operation.
Basically, in particular a firing, debinding and/or sintering process may be divided into several subsections, for example into a drying process or debinding process, respectively, followed by the actual firing or sintering process, respectively, which in turn is followed by a cooling process.
Drying processes are usually conducted in a furnace which still is completely open or partially open and the same applies to cooling processes.
This is especially important when processing still moist dental ceramics caused by manufacture (e.g. layering ceramics, glazes, glass solders etc.) or ceramics in which heating gradients for preventing damages for example due to cracks, bubbles, porosities, an incomplete evaporation or burning of organic materials and/or heating and cooling gradients for limiting tensions within the ceramics itself, or damages due to different temperature gradients between different framework materials (metal or ceramics) and the applied coating materials or veneers, are important.
By default today's devices work with settings that have been empirically determined. The closing or opening of the firing hood is effected in a given process, e.g. within a given time and by means of a given closing movement, e.g. linear or via fixed positions. Thereby, the firing hood can already be heated to a predefined temperature or may freely cool down.
It has already been proposed to use empirically validated models according to which for example based on the temperature measured within the furnace head the thermal energy and the heating to be expected can be estimated as a function of the distance between the hot firing hood and the dental object, and the closing movement is adjusted based on the experimentally determined models.
Moreover, it has been proposed to use thermal elements that are located in the furnace head or furnace base, namely next to the working area of the dental restoration parts to be processed, but that nevertheless cannot exactly represent the temperature of these objects. It is difficult to align these sensors according to the facts. A thermal element would have to be attached to a dental object which is not feasible in practice.
Finally it has also been proposed to direct an optical temperature instrument towards the object, for example with the aid of a beam of light (laser pointer), in order to achieve a representative measurement value of the temperature of the object. The alternatives described exhibit the disadvantage that an incorrect attachment of the sensor and/or the dental object may result in a large measurement error. Moreover, the attachment of the sensors is partially difficult, for example if the object has a powdery consistency. Moreover, the measurement position of the sensors is not obvious to the user/operator, i.e. usually the dental technician, which is especially true when several dental restorations and/or larger dental restorations are to be processed.
In view of the existing problems and also the liability risks due to incorrectly burned dental restoration parts, the burning process is normally carried out with considerable safety reserves, namely with extended hold times, during which the temperature level should adjust. The definition of extended hold times here is partially left to the user. On this occasion, however, a deterioration of the properties of the ceramics used for the firing of the dental restoration part is accepted.